Brake system with a new type of mux control (mux 2.0), having an outlet valve per brake system or an outlet valve per brake circuit, and method for controlling pressure

ABSTRACT

A brake system for motor vehicles may include an actuation device (e.g., brake pedal), a travel simulator to generate a feedback force on the actuation device, a first piston-cylinder unit having at least one piston that separates two working chambers that are connected via at least one hydraulic line to at least one wheel brake of a brake circuit, a control device and a pressure supply unit driven by an electric motor. At least one wheel brake may be assigned to each brake circuit, and each wheel brake may be connected to its associated hydraulic connecting line via a controllable switching valve. An outlet valve may be assigned to a single wheel brake or to a single wheel brake of each brake circuit in a hydraulic connection between the wheel brake and a pressure medium storage container, without any further valve disposed as such.

The invention relates to a brake system for a motor vehicle according tothe preamble of claim 1.

PRIOR ART

WO2006/111392A1 and WO2010/091883 A1 disclose brake systems in which, inABS mode, the pressure in the wheel brakes is adjusted simultaneously orsuccessively in a closed multiplex process. This takes place via aswitching valve and via the position-regulated control of a drivenpiston for pressure build-up and pressure reduction taking into accountthe pressure-volume curve of each individual wheel brake. Preferably,switching valves with low flow resistance are used in connection to thewheel brake. The pressure can here be changed in one or more wheelbrakes sequentially or simultaneously. For control, a pressure sensor isused which measures the pressure in the hydraulic connection between thepiston-cylinder unit and wheel brakes.

The advantage of this method is a very precise pressure regulation, inparticular with low friction coefficients and in recuperation. Also, thecost of the valves can be significantly reduced, since only oneswitching valve is required instead of one inlet and one outlet valveper wheel brake. The disadvantage with the brake systems known fromthese two documents is the high requirements for the electric motor.This must amongst others have a low inertia mass and a high torque forreversed operation.

DE 10 2012 002 791 A1 discloses a wheel brake, the basic structure ofwhich with a master brake cylinder and isolating valves is known in themarket as MKC1, see also DE 10 2013 224313 A1. Multiplex mode here isconfigured such that both the master brake cylinder and the pressuresupply unit are connected to the brake circuits via respective isolatingvalves.

The advantage of this arrangement is the modular structure and the useof standard components (master brake cylinder), and the use of aseparate pressure supply unit. In this arrangement, no differentialpressures occur in the brake circuits since the pressure supply unit isconnected to the brake circuits via isolating valves, and nodifferential pressures occur due to the interconnection of a piston formedia separation.

The disadvantage however is the high cost of components. Amongst others,a large number of valves is required, with a complex master brakecylinder with two chambers and a simulator.

DE 102014117727 supplements the brake system described in DE 10 2012 002791 A1 with a novel pressure supply unit which has a double-strokepiston operated in advance and return strokes, and in advance stroke hasa different hydraulic cross-section area than in return stroke, and withwhich a controlled pressure reduction is possible.

The advantage of this configuration is the continuous delivery by thepressure-generating unit, and the motor-downsizing potential inconventional brake systems with inlet and outlet valves, by the use of asmaller hydraulic area. Because of the high dynamic requirements formotor torque for use in regulating mode, the advantage of downsizingcannot however be utilised and hence the brake system cannot beadvantageously minimised.

Furthermore, various master brake cylinder designs with travel simulatorare known from the prior art which are constructed with two pistons orthree pistons and a travel simulator.

Advantageous designs of the master brake cylinder provide correspondingvalve circuits for the travel simulator (shut-off in fall-back level,function valves, infeed on fall-back level), and isolating valves forthe brake circuits for decoupling the pedal in brake-by-wire mode.Reference is made here merely as an example to DE 10 2010 081463 A1 andDE 10 2013 216477 A1. Further brake systems are known from DE 10 2015103859.5 (MUX with two pistons), DE 102011102270 (three pistons), DE 102013216477 A1 (CAS, 3-piston system, auxiliary piston, push-rod piston,floating piston with partial MUX mode) and DE 10 2013224313.

The pressure regulating module described in this invention functionswith all the above-mentioned designs for brake-by-wire master brakecylinders with travel simulator, and is therefore not explained in moredetail below. Differences in the master brake cylinder design occursubstantially because there are different customer preferences withregard to pedal feedback, and automotive suppliers wish to use standardcomponents in the master brake cylinder, and certain designs require oneor more isolating valves for the pressure supply unit.

DE10 2013 216477 A1 describes a three-piston THZ with valve circuit forthe pressure supply unit and pressure regulation for ABS. In normalmode, the second pressure chamber is pressureless and the third isassigned to the SK piston. This locks in its starting position. Thepressure regulation takes place in the HA circuit with MUX, and in thefront axle circuit either with MUX or two additional outlet valveswhich, in P_(ab) function, conduct the pressure medium to the storagecontainer via an additional valve. Pressure regulation in multiplex modedoes not take place via the volume measurement known fromWO2006/111392A1, but via PWM of so-called wheel valves with constantpressure measurement by means of pressure sensors.

OBJECT OF THE INVENTION

The object of the present invention is to provide an economic brakesystem with high regulation quality and regulation performance.

ACHIEVING THE OBJECT

The object of the invention is achieved with a brake system with thefeatures of claim 1. Advantageous embodiments or designs of theinvention arise from the features of the dependant claims.

The brake system according to the invention is distinguished by asignificant improvement in comparison with previously known brakesystems with multiplex regulation, which are configured with 4 switchingvalves or 8-valve technology with inlet and outlet valves for ABS. Thebrake system according to the invention can advantageously be useduniversally in combination with different designs of master brakecylinder of a brake-by-wire system.

The brake system according to the invention is distinguished by a highlydynamic MUX mode and allows a great improvement in performance and greatreduction in cost due to the minimal number of valves, whereinadvantageously simple switching valves based on modified inlet valvesmay be used. Also, only a few pressure emitters/sensors are required. Itis of particular advantage that only a small low-cost motor is requiredas a drive for the pressure supply unit.

The invention advantageously provides a pressure regulating module withpressure regulation which is distinguished by high pressure regulationquality, high dynamics thanks to short cycle time, particularly simpledesign of the low-flow switching valves with advantageous contact flow,a reduction in the requirements for the motor of the pressure supplyunit, and minimal flow resistance.

This is achieved with a compactly constructed brake system which expandsthe advantages of a multiplexer in the form of high regulation qualityin pressure regulation by pressure-volume control in various operatingmodes such as e.g. recuperation, ABS, ESP, ASR, by novel pressurereduction and pressure build-up regulation concepts which require fewoutlet valves to reduce the cycle time in the temporarily open brakecircuit.

The requirements imposed are fulfilled according to the invention byoperation in the closed and partially open brake circuit with minimalnumber of valves, and preferably an advantageous design of thepressure-generating unit with intelligent construction of thepressure-generating unit with a pressure piston delimiting only oneworking chamber, or a double-stroke piston which delimits two workingchambers.

The following basic concepts form the background to the brake systemaccording to the invention:

-   -   pressure regulation in the closed and partially open brake        circuit with minimal volume loss in ABS regulation mode;    -   pressure build-up and pressure reduction with many degrees of        freedom, high regulation quality and dynamics with switching        valves and only one outlet valve in just one or both brake        circuits;    -   partially simultaneous pressure build-up and pressure reduction        in the closed brake circuit using a double-stroke piston in the        pressure supply unit;    -   use of current measurement of the electric motor of the pressure        supply unit for indirect pressure measurement, and use in        particular in the pressure reduction and pressure build-up        regulation in two brake circuits;    -   more precise pressure-controlled pressure reduction by means of        pressure measurement via pressure sensors via the pressure        supply unit and valves which connect the pressure supply unit to        the storage container (double-stroke piston with PD1, PD3        valve),    -   novel design of the pressure supply unit in the embodiment as        double-stroke piston with pre-fill effect, use of different        hydraulic surface areas in particular on pressure build-up at        high pressures;    -   minimising of operation in the open brake circuit by preferably        regulation in multiplex mode in the closed brake circuit.

In the brake system according to the invention, evidently not all theabove-mentioned ideas need be implemented, but it is essential that onlya maximum of one outlet valve is provided per brake circuit, and oneswitching valve per wheel brake for pressure build-up and pressurereduction, whereby advantageously the number of necessary valves is lessthan the eight which are required with a conventional ABS system.

The brake system according to the invention also has a novel intelligentmultiplex method which provides a largely simultaneous pressurereduction in several wheel brakes via time control of the outlet valveor valves, and optionally allows also simultaneous pressure reductionand pressure build-up in different brake circuits.

The invention is based on the concept that in normal operation with lowdynamic requirements and high pressure-setting precision requirement, inparticular with normal brake force amplification, recuperation, ABS atlow μ, in all wheel brakes or wheel brake cylinders, the pressure isbuilt up and reduced simultaneously or sequentially via travel controlof the piston of the pressure supply unit, taking into account thepressure-volume curve(s). Here no PWM control of the switching valves isused, or a simplified valve circuit. Instead, the switching valvesassigned to the wheel brakes on pressure setting are always open for theentire time and closed after reaching the desired or predefined nominalpressure, in order to maintain the brake pressure in the wheel brakes.In operating situations with high dynamic requirements such as e.g. ABSat high μ, μ split, ESP and ASR, the pressure in principle is alwaysbuilt up in all wheel brake cylinders with pressure-volume control inmultiplex mode, i.e. simultaneously or sequentially. Here again, thereis no PWM control of the switching valves, but the pressure is reducedin some of the wheel brakes in multiplex mode simultaneously orsequentially, while in the one or both wheel brakes the pressure istransferred to the storage container via the respective assigned outletvalve, wherein the respective outlet valve is opened only for apredefined time so that during this time, the pressure in the wheelbrake can fall to the nominal pressure. Also, the pressure reduction cantake place via a working chamber of the pressure supply unit, and fromthere via a switching valve to the storage container. The switchingvalve is also time-controlled so that in the predefined time in whichthe valve is open, the pressure can fall to the nominal pressure. Thepressure reduction in the other wheel brakes may take place at the sametime via the volume control by means of the piston of the pressuresupply unit.

The pressure fall regulation in multiplex mode in the closed brakecircuit is extended in comparison with the prior art in that, onsimultaneous pressure reduction in two wheel brakes, the switchingvalves SV are open simultaneously or with a temporal offset, wherein theswitching valve of the wheel with the higher pressure is opened early.

The pressure is reduced in the open brake circuit preferably by timecontrol of the outlet valves to the storage container. By pressurereduction via outlet valves, the brake circuit is opened for ashort-time.

Thanks to the above extensions, the load on the multiplexer or thepressure supply unit can be greatly reduced and at the same time theregulation quality increased thanks to shorter cycle times.

Thus, the pressure in one brake circuit can be reduced rapidly in thatthe pressure is reduced by opening the outlet valve of the one wheelbrake, and at the same time the pressure in the other wheel brake of thebrake circuit is reduced by means of the pressure supply unit. In aconventional multiplexer without corresponding outlet valve, thepressure reduction in the two wheel brakes of a brake circuit would haveto take place temporally successively, and thus take at least twice aslong.

Also, advantageously the outlet valve can be used for the pressurereduction in both wheel brakes of the brake circuit if both the outletvalve and the two switching valves assigned to the wheel brakes areopened in multiplex mode.

Advantageously, only one outlet valve is used in a brake circuit, inparticular to simplify the regulation in brake circuit II. Thus, oneoutlet valve (AV3, FIG. 1b ) is used to reduce the pressure in one ortwo wheel brake cylinders. For reduction in two wheel brakes via oneoutlet valve, the wheel brake cylinder is isolated from the pressuresupply unit. At the same time, the pressure in brake circuit BK I and BKII can be built up or reduced simultaneously or sequentially bypressure-volume control of the wheel brakes. This degree of freedomleads to a significant reduction in the cycle time of the pressurisationof four wheel brakes, and has a highly advantageous effect on theregulation quality (deviation of wheel speeds from vehicle speed) inparticular in extreme situations, e.g. changing μ, in particular in thehigh μ range, and leads to shorter braking distances. Also, the volumeloss in the brake circuit is minimised because in regulation mode, onlyvery little volume is lost. This has advantageous effects on a smalldimensioning of the volume of the pressure supply unit.

The proposed full MUX systems of the prior art are known to have theproblem of simultaneous pressure fall P_(ab) when the pressure level inthe wheel brakes differs greatly. Many use the volume control to controlthe pressure by using the pressure-volume curve of the wheels/wheelcylinders. The time loss on pressure fall p_(ab) of the wheel/wheelbrakes, in particular on the front axle, should however be as low aspossible. The switching times of the known MUX systems however cause asignificant time shift due to the switching time required in multiplexmode. Because of the high brake force proportion on the front axle (V),this is particularly susceptible to good regulation, which means thatthe wheels must always be operated with high dynamics and almost optimalbrake pressure close to the slip optimum. The brake system according tothe invention meets these demands particularly well if the high dynamicregulation strategy described above with volume and time control isused.

Furthermore, a (minor) disadvantage with the previously known brakesystems described above is that no pressure build-up p_(auf) can takeplace if one wheel required a pressure fall p_(ab). As an alternative tofull MUX, partial MUX systems are proposed in which one BK is designedwith MUX and the other BK with conventional inlet and outlet valves.

One essential disadvantage of the outlet valves (AV valves) is thepoorer precision of the pressure control, pressure fluctuations andnoise formation. With the regulation strategy described above, primarilythe quiet volume-controlled multiplex mode is used. The outlet valvesare not required continuously and are used relatively rarely.

This is achieved by priority control of the multiplexer with firstpriority for the front axle and additional use of the outlet valve AV inunchoked return to the storage container. For the rear axle, thepressure reduction p_(ab) takes place with second priority in themultiplexer. Alternatively, a pressure fall p_(ab) may take place on therear axle with precise time control of the switching valves, wherebyonly a slight time delay occurs.

Throughout the entire regulation process, at the start of the pressurereduction, the pressure in all wheels is stored so that the MUX controlsystem of the pressure supply unit (DE) can immediately switch tooptimal control pressure. Precise time control of the outlet valves, incomparison with the prior art, is possible because the pressuredifference is known, and from the pressure-volume curve the volume andhence the throughflow quantity for time control of the outlet valve or aswitching valve can be determined. Pressure fluctuations towards the endof the pressure change can also be reduced by supporting the pressurereduction with a corresponding piston control of the pressure supplyunit.

The temporal sequence of the method is shown and explained later indetail in the figures.

The term volume control according to the invention means that thecontrol device evaluates the current pressure levels, thepressure-volume curves and the target nominal pressures for therespective wheel brakes, and using these data calculates the necessarydelivery volume which must be supplied by the pressure supply unit. Fromthis delivery volume, the necessary movement travel of the piston of thepressure supply unit can be determined. With corresponding valvecircuits and design of the pressure supply unit, it is possible toimplement a pressure build-up P_(auf) in one brake circuit and at thesame time a pressure reduction p_(ab) in another brake circuit.

Previously, the valves required for multiplex operation were more costlybecause of the requirements for differential pressure and flowcross-section due to the large dimensioning of the magnet circuit. Bycorresponding supply to the magnetic valve from the brake circuit intothe armature chamber, and then via the valve seat to the wheel cylinder,with the brake system according to the invention, advantageously, alow-cost standard magnetic valve can be used.

To support the regulation, a pressure sensor is used to determine thepressure in one brake circuit. The pressure in the other brake circuitcan be determined indirectly on separation by an isolating valve, usingthe known method of phase current measurement of the electric motor. Theaccuracy of the pressure estimate is increased further if a temperaturesensor is arranged in the electric motor driving the piston, since thetorque constant changes proportionally to the temperature. With theknown cross-section area of the master brake cylinder and gearreduction, the pressure can be calculated by the proportionalcorrelation between the phase current and torque of the electric motor.

An advantageous feature of the brake system according to the inventionis the use of a double-stroke piston with which the pressure can bebuilt up and reduced in multiplex mode. It is also advantageous if afurther valve (TV2 b, or ZAV) is available for pressure reduction in thesecond chamber (4 a, FIG. 5) of the double-stroke piston. In particularwith the double-stroke piston, the use of one or more pressure reductionvalves (PD3, PD1 FIGS. 5a -5 d, FIG. 6) is useful, so that the pressurein the brake system can be largely reduced in closed mode and pressurereduction can take place with low noise even at high pressures. This isdesirable in particular in brake servo mode with fading and pressurereduction when stationary after ABS intervention.

The pressure is then reduced either via the piston return stroke, thepressure-regulated pressure fall by means of pressure measurement viathe pressure sensor for the pressure supply unit, and/or via valveswhich connect the pressure supply unit (double-stroke piston) to thestorage container (i.e. PD3, PD1). The pressure sensor in brake circuitBK II is used to regulate the pressure reduction in both brake circuits.If the pressure is to be reduced individually in brake circuits I andII, in addition the pressure estimate based on the phase currentmeasurement is used. The pressure reduction can here take place via onechamber 4 of the piston or both chambers 4 and 4 a.

In normal brake servo mode, the pressure is reduced via the pistonreturn stroke to pressures close to the blocking pressure, the pressurefall via PD3, PD1 on pressure reduction from high pressures, inparticular after fading or at the end of ABS control processes.

The double-stroke piston in the pressure supply unit may be configuredsuch that the hydraulic surface areas in the advance and return strokesare different. By changing the hydraulically active areas, the torquerequirement at high pressures is reduced. At the same time, a pre-filleffect can be achieved, i.e. by a larger volume flow at low pressures, avery rapid brake application can be achieved or a pad clearanceovercome.

The small hydraulically active area is effective because thedouble-stroke piston is operated in return stroke, or additionally, inadvance stroke, the front and rear chambers of the double-stroke pistonare connected via a changeover valve (ShV) or two valves (TV2 and TV2b), and hence a small hydraulic area acts in pressure build-up. If thedouble-stroke piston is retracted, by opening a pressure reduction valvePD1, the pressure in both brake circuits can be dissipated into thestorage container. Thus low-noise operation in closed brake circuit ispossible. By intelligent activation, the opening of the isolation valvecan be supported even at high differential pressures (brake circuitpressure to pressure in the double-stroke piston) if the double-strokepiston changes the pressure in the working chamber before opening of thevalve, and allows opening at low differential pressures. This allows adownsizing of the isolating valve, in particular its design for highthroughflow and low differential pressures.

The pressure reduction then takes place either via the piston returnstroke (PD1 open) via pressure-volume control and if necessary openingof outlet valves in brake circuit II and pressure reduction via TV2 b(ZAV). For noise reduction, an outlet valve can be openedtime-controlled for this operating point and the pressure fallinfluenced by the piston, so that pressure fluctuations are avoided anda gentle swing towards the target pressure level is achieved. This maybe used effectively in particular on pressure fall via ZAV.

In the return stroke with valve PD1 closed, pressure can only be builtup or the volume of one brake circuit shifted to the other. Thispressure build-up is preferably used only if the pressure must be raisedsignificantly above the normal operating level, such as e.g. onfading >120 bar.

Also, pressure can be reduced in one brake circuit in one or two wheelbrake cylinders, and at the same time pressure built up in the otherbrake circuit with the pressure-volume control method as depicted inFIG. 5b . The pressure is regulated via the correspondingly adaptedpressure-volume curve, which takes account of volumes of the activatedwheel brakes and the hydraulically active area. When the target pressureis reached, the associated valve is closed and the volume of the wheelbrake still active is reduced via PD1. At the same time, a pressurereduction is possible via an outlet valve.

In the system, preferably MUX regulation is used, i.e. pressure controlvia the pressure-volume curve with closed brake circuit (FIGS. 2a-2b ).Thus, in the closed brake circuit, the pressure can be built up andreduced on the basis of the pressure-volume curve. This takes placemainly with brake force amplification, recuperation, ABS mode in lowfrequencies and pressure amplitudes. In other operating cases, e.g.controlled pressure fall after ABS operation, simultaneous pressurebuild-up and pressure reduction at high frequencies, the pressure can beinfluenced in pressure reduction in addition to time control of theoutlet valves, if necessary supported by plunger travel control (FIG. 6b).

After the pressure reduction on opening of the brake circuit, there isalways a volume loss in the brake circuit and hence a change in travelin the piston position of the pressure-generating unit. Therefore, it isuseful to detect the offset shift As, of the pressure-volume curve (FIG.2b ). This is not necessary for MUX regulation, but for regulation andoptimising of the volume balance in order to prevent the piston fromtravelling to a stop on a regulation process. In particular on use of adouble-stroke piston with limited volume in one stroke direction,information on the absolute position of the piston is important forregulation.

This proposed control with outlet valve(s) and pressure reduction p_(ab)time control substantially relieves the load on the motor dynamics. Therare use of the outlet valve brings the advantages that the ABS pressuremodulation does not necessitate opening of the brake circuit, whichlowers the probability of a brake circuit failure and entails particularadvantages for autonomous driving/braking.

The pressure regulation module with pressure regulation and its variousembodiments thus offers a module for perfect pressure regulation withoutrestriction and high safety on faults. The disadvantages of theconventional multiplexer—e.g. long cycle time from sequential wheeloperation, no possibility of simultaneous pressure build-up andreduction, high requirements for electric motor dynamics—are thuseliminated and form the basis for an almost perfect regulation withminimum valve complexity. Depending on choice of pressure supply unit(single-stroke piston or double-stroke piston), different degrees offreedom are possible. The single-stroke piston has the advantage of lowsoftware complexity; the double-stroke piston offers all degrees offreedom and motor-downsizing potential. Also, independently of thechoice of pressure supply unit, the requirements for motor torque forreversed operation of the multiplex regulator are drastically reducedand the size and cost of the electric motor can be significantlyreduced.

A further improvement in the system layout advantageously results frominfeed of the volume of the pressure supply unit via a blow hole intothe front side of the floating piston. This also advantageouslyincreases safety with simultaneously reduced cost. With this systemlayout, the isolating valve TV1 may be omitted since the pressure supplyunit is isolated via movement of the piston on system failure. Thisoffers cost advantages (fewer valves) and reduces the flow resistancebetween the pressure supply unit and the first brake circuit (BK1).

DESCRIPTION OF THE DRAWINGS

FIG. 1 a: shows a first possible embodiment of the brake systemaccording to the invention with master brake cylinder, pressure supplywith outlet valve(s) in one or two brake circuits;

FIG. 1 b: shows an example of a simplified pressure-volume curve;

FIG. 1 c: shows pressure regulation options in the basic system of FIG.1 a;

FIG. 1 d: shows valve circuit with AV/EV in the regulation systems ofthe prior art;

FIG. 1 e: shows advantageous valve circuit for new regulation systemwith switching valves and an outlet valve in one brake circuit;

FIG. 1 f: shows inlet valve according to the invention in the brakecircuit;

FIG. 2a : shows pressure-volume control in a closed brake circuit (AV,ZAV closed);

FIG. 2b : shows pressure-volume curve for wheel brake 1 and wheel brake2 with offset shift by opening of brake circuit;

FIG. 3: shows conventional multiplex regulation in sequential order;

FIG. 3a : shows cycle shortening of multiplex regulation with AV valvein pressure reduction;

FIG. 3b : shows time control on pressure reduction via outlet valve;

FIG. 4: shows temporal development of an exemplary regulation with 4wheel brakes;

FIG. 4a shows temporal development of an exemplary regulation with 4wheel brakes;

FIG. 4b : shows temporal development of an exemplary regulation with 4wheel brakes;

FIG. 5: shows advantageous brake system structure with double-strokepiston (DHK);

FIG. 5a shows pressure build-up regulation in multiplex mode accordingto the invention with DHK and outlet valve;

FIG. 5b shows simultaneous pressure build-up and pressure reduction inmultiplex mode in DHK return stroke and outlet valve;

FIG. 5c shows simultaneous pressure build-up and pressure reduction inmultiplex mode in DHK advance stroke and outlet valve;

FIG. 5d shows regulated or controlled pressure reduction in both brakecircuits in closed circuit via pressure-generating unit and PD3 valve;

FIG. 6: shows double-stroke piston system in advantageous dual circuitdesign.

DESCRIPTION OF THE FIGURES

FIG. 1a describes the basic embodiment of the brake system according tothe invention with master brake cylinder HZE, pressure supply unit DEwith single-stroke piston (3) and outlet valve(s) AV1, AV3 in one oroptionally two brake circuits. Brake circuit II is advantageouslyassigned to the front axle. The outlet valve AV1 is optional, i.e. neednot necessarily be provided.

The brake system consists of a master brake cylinder according to theprior art, comprising a master brake cylinder unit HZE, floating pistonSK with return spring 1, a pressure piston DK or ram or an auxiliarypiston HiKo, a hydraulically actuated travel simulator WS andcorresponding control valves HZV for the function of the piston-cylinderunit, as described for example in the prior art.

The following embodiments amongst others are possible:

-   -   a) master brake cylinder with two pistons in the form of a        pressure piston DK and a floating piston SK with connected        travel simulator which can be shut off via a valve,    -   b) 3-piston system with auxiliary piston HS for travel simulator        actuation and infeed valve and/or mechanical intervention in the        event of a fault,    -   c) 2-piston system with floating piston SK and auxiliary piston        HiKo with infeed.

In all embodiments, the master brake cylinder unit HZE can be isolatedfrom the pressure supply unit DE. According to variant Var2, this can beachieved via isolating valves TV1 and TV2, or in the second variant Var1shown, via blocking of the supply of the floating piston. The valvecircuit of the HZE ensures that no undesirable feedback occurs to thepedal BP when the pressure supply unit DE is active, and in fall-backlevel (system failure) the volume of the master brake cylinder unit HZEis guided to the wheel brakes RB1-4. Also, a switching valve SV1-4 isarranged for each wheel brake in the hydraulic connection to therespective associated working chamber A1 or A2 of the brake mastercylinder HZE. The concrete embodiment of the master brake cylinder HZEis not however relevant for the brake system according to the invention.

The brake system has four switching valves SV1, SV2, SV3 and SV4, viawhich the pressure supply DE and the master brake cylinder HZE areconnected to the wheel brakes RB1-4. The switching valves SV1-4preferably have a low flow resistance and are suitable for MUXoperation. In addition, an outlet valve AV3 is provided in a brakecircuit for pressure reduction in the wheel brake in RB3 independentlyof the MUX, and is arranged in the hydraulic connection between thewheel brake RB3 and the storage container 10. Preferably, the outletvalve AV3 is positioned on the front wheel brake RB3 of a brake circuitsince, in extreme cases, the pressure in this wheel brake must bereduced quickly and without great time delay because the significantbraking effect originates from the front axle.

The pressure supply unit DE consists of an electric motor M, which via aspindle 2 drives a piston 3 which compresses or shifts the volume in thepressure chamber 4. The motor M of the pressure supply unit may comprisetwo or three sensors: a) angle sensor 6, b) current measurement sensorfor measuring the phase currents of the electric motor 7, and c) ifnecessary, a temperature sensor 8 for determining the coil temperatureof the electric motor M.

The pressure-generating unit DE is preferably arranged in the valveblock or HZE. The pressure chamber 4 of the pressure-generating unit DEis connected to a storage container 10 via a check valve 5. A pressuresensor 9 is arranged at the outlet of the pressure-generating unit DE.The brake circuit II is connected via the isolating valve TV2, and brakecircuit I via the isolating valve TV1, to the pressure supply unit DE.The isolating valve TV1 may be omitted if one chamber is separated bythe pressure supply unit DE in fall-back level. This can be achieved bya pressure infeed from the pressure-generating unit DE via the blow holeSL of the floating piston SK.

For pressure modulation in ABS and recuperation, the control device andits regulator determine the necessary pressure change for pressurebuild-up (referred to below as P_(auf)) and pressure reduction (referredto below as P_(ab)). The pressure is regulated by thepressure-generating unit DE, in that simultaneously or with a temporaloffset, the individual wheels/wheel cylinders are supplied withpressure. For this, the electric motor M shifts the corresponding volumefor pressure change in both directions via e.g. the piston 3.

Here, the pressure change according to the prior art can be modified bycorresponding time control with PMW of the switching valves and pressurecontrol of the pressure of the DE. This however requires a very precisePWM process with complex pressure model. Preferably, therefore, thevolume control is used as already described above. For this, the data ofthe pressure-volume curve (p-V curve—see FIGS. 1a and 2a ) of therespective wheel brake RB1-4 involved in the pressure build-up orreduction are stored in the memory of the regulator unit. If now theregulator requests a pressure change Δp, for pressure regulation at thewheel the differential volume ΔV is adjusted accordingly by the pistonin both directions +ΔS. For this, one or more switching valves areopened which are closed again after completion of the volume shift. Theposition of the piston 3, e.g. at the start, middle or end of thestroke, is irrelevant for ΔP volume control for the regulation. Here,during the pressure change, a temporal control may be used in order toimplement transition functions towards the end of the pressure change,e.g. to reduce the pressure fluctuations and the associated noise.

A high dynamic is important if two or more wheels require a pressurechange simultaneously. For this, the invention proposes that to relievethe load on the motor dynamics, one or two additional outlet valves AVare used. For volume control, in particular also the pressure level inthe pressure-generating unit DE and in the wheels is important. It isfavourable here that the pressure level on pressure change correspondsto the starting pressure of the wheel to be regulated. This achieves arapid and low-noise pressure regulation. The temporal developments areillustrated in FIGS. 3, 3 a, 3 b and 4, 4 a, 4 b. Suitablepressure-generating units DE are all pumps with single piston, steppedpiston, double-stroke piston and also e.g. gear pumps which allowprecise volume control.

In FIG. 1 a, for the above-mentioned functions, the pressure generationtakes place in one circuit directly via an isolating valve TV1 in BK1(Var2) or alternatively via a blow hole SL on the front of the SK piston(Var1). The pressure is supplied to brake circuit BK2 via an isolatingvalve TV2. For infeed via the blow hole SL on the front of the floatingpiston SK, optionally the isolating valve TV1 may be omitted since, onsystem failure, the pressure-generating unit DE is isolated from themaster brake cylinder effect because piston SK moves and shuts off thepressure supply DE. Alternatively, as drawn in dotted lines, thepressure-generating unit DE may be connected directly to BK1 via TV1(Var2). Since the SK piston in Var1 is only moved in fall-back level, aspecial diagnosis circuit is required in which the floating piston SK ismoved and checked for tightness.

FIG. 1b describes the known pressure control based on a simplifiedpressure-volume curve which forms the basis of the MUX regulation.Depending on the required pressure difference Δp, from the curve avolume change ΔV is read which is implemented as a travel change As ofthe piston 3 by shifting the plunger of the pressure-generating unit DE.This applies for both pressure build-up and pressure reduction.

FIG. 1c shows a fundamental possibility for pressure regulation in thebasic embodiment of FIG. 1. The system itself has the following degreesof freedom in pressure regulation:

-   -   pressure build-up and pressure reduction in all brake circuits        BKI and BKII mainly with multiplex regulation (pressure        regulation with pressure-volume control) in all wheel brake        cylinders simultaneously or sequentially;    -   multiplex regulation in pressure build-up and pressure reduction        in brake circuit I via the opened isolating valve TV1 and        simultaneous pressure reduction in brake circuit II via the        outlet valve AV3;    -   multiplex regulation via SV1, SV2, SV4 in pressure build-up and        pressure reduction in brake circuit I and II for wheel brakes        RB1, RB2 and RB4, and simultaneous pressure reduction in wheel        brake RB3 via opened outlet valve AV3 with closed switching        valve SV3.

The pressure reduction p_(ab) via switching valves SV1 and SV2 in BK Itakes place mainly via pressure-volume control, sequentially orsimultaneously. For this, the respective switching valve SVi is alwaysopened. For simultaneous pressure reduction p_(ab) at different startingpressures, optionally by deviation from the MUX regulation, switchingvalves SV1 and SV2 may be opened with a time offset, and pressurereduction p_(ab) controlled via a switching valve SV2. The isolatingvalve TV1 is always opened on pressure reduction. In this exemplaryembodiment, the wheel brake RB1 has a higher pressure, therefore theassociated switching valve SV1 is opened before the switching valve SV2.On the basis of knowledge of the pressure difference—the pressures inwheel brakes RB1 and RB2 and the pressure in the pressure-generatingunit DE are known—the time control may be dimensioned precisely. SV2 isopened when the pressure in the pressure-generating unit DE isapproximately reached. Further pressure reduction then takes placesimultaneously in both wheel brake cylinders RB1 and RB2 by control viapiston 3 when switching valves SV1, SV2 and TV1 are open. When thetarget pressure of a wheel is reached, the corresponding switching valveSV1 or SV2 is closed. If further pressure reduction is desired in onewheel, the further pressure reduction can take place in the respectivewheel brake.

As already described, to simplify the system, preferably the PWM controlis omitted, in particular also for noise reduction.

Exemplary temporal curves of the pressure reduction are described inFIGS. 4a to 4 c.

FIG. 1d shows a conventional valve circuit for ABS with four inletvalves EV and four outlet valves AV. If this is also used with fewer AVfor MUX, e.g. with differential pressure in MUX, as well as thispressure the fault case must be taken into account in which, on anasymmetric road surface, the pressure-generating unit and also the valveactuation fail suddenly e.g. due to the ECU, and at the same time thepressure-generating unit has a low pressure level. In this case forexample, EV1 has 130 bar and EV2 0 bar. On failure of thepressure-generating unit, this means that for EV1 the return spring ofthe valve armature must open around 130 bar. To enable this, the magnetcircuit of the valve must be sufficiently large, whereby the valvebecomes costly. Alternatively, a pressure-relieved valve may be used,but its costs are also high.

In dimensioning of the valve seat, it must also be taken into accountthat this should be as large as possible in order to generate a smallbackup pressure if the brake pressure is to be built up rapidly by thepressure-generating unit. The backup pressure is introduced directlyinto the motor torque or power.

FIG. 1e shows a modified throughflow of the switching valves SV. Thehydraulic medium flows from the brake circuit or pressure-generatingunit through the armature chamber to the valve seat and onto the wheelcylinder. If the above fault case occurs, the wheel pressure opens theswitching valve. The magnetic force must however also close around 130bar, which takes place however with a small armature air gap in thevalve end position. The return spring of the switching valve SV needtherefore only be slightly strengthened so that the switching valve doesnot “snatch” at correspondingly high volume flow. Since conventionalinlet valves must close at around 220 bar—in FIG. 1 e, 130 bar—, withthe same magnet dimensioning the valve seat area may be increased whichmeans a smaller backup pressure or flow resistance and is advantageousfor MUX mode. The valve circuit depicted in FIG. 1e is thereforeadvantageous for the brake system according to the invention.

FIG. 1f shows a possible embodiment of the inlet valve EV according tothe invention and the connection to the brake circuit BK and thepressure supply DV and wheel brakes RBi.

The inlet valve EV has a magnet armature MA, a magnetic base body MGKand an exciter coil ES. When the magnetic valve EV is powered, themagnet force MK shifts the armature MA from position S_(A0) to positionS_(A2) by the differential travel S_(A). The magnet armature MA moves aram MStö by the same travel, so that the ram MStö comes to rest on thevalve seat VS and closes the outlet Ea of the magnetic valve. Thearmature MA at this point still has a residual air gap S₀ from themagnetic base body MKG, which is provided so that the armature MA doesnot stick to the magnetic housing MGK when the power to the exciter coilES of the valve EV is switched off, due to re-magnetisation losses ofthe iron circuit. When the valve current is switched off, the returnspring RF moves the armature MA back to the starting position. Themagnet force F_(M) rises nonlinearly with a smaller air gap, e.g. withincreasing travel. The return spring F_(RF) is dimensioned such that themagnetic force F_(M) in the starting position S_(A0) is greater than thespring force, so that a secure closure of the valve is guaranteed. Thespring force increases with the increasing travel S_(A) and in the endposition S_(A2) is however smaller than the magnet force F_(M).Preferably, a linear spring is used, so that the magnet force F_(M) inthe end position for a given current is significantly higher than thereturn force, so that the valve can be retained with low current, orsecure closure is guaranteed even at high differential pressures betweenthe wheel brake and the pressure supply. This retention is also ensuredat high differential pressures since the magnet force increases stronglynonlinearly at the closed valve position. The return spring must howeveralso be dimensioned such that the function as an unpowered open valvecan be ensured and the valve always opens safely.

The outlet E_(a) of the valve is connected to the wheel brakes RBi(RB1-RB4), the inlet E, to one brake circuit BKi or to the pressuresupply unit DV (20). With such connections, the inlet valve EV can beopened both by the return spring RF and by the pressure in the wheelbrake, which is very important in particular in the event of a fault ormalfunction in the brake system (e.g. loss of voltage to the valve).Also, even at high pressures in the brake circuit and small pressures inthe wheel brake, only the pressure difference between inlet Ei andoutlet Ea acts on the ram MStö. This differential pressure at the valveis relatively low in pressure build-up, but must however be taken intoaccount in the spring design RF so that the pressure difference does notlead to the valve being pushed back on pressure build-up when the volumeof the pressure supply DV is delivered to the wheel brake. Valves withlarge opening cross-section ÖQ or low flow losses reduce this effect.

In particular on pressure build-up with pressure-volume control or timecontrol with low differential pressure between the pre-pressure andactual pressure in the wheel brake, the valves described above withlarge opening cross-section may be used since the regulation accuracy isvery high. This in turn has advantages in that only low flow lossesoccur, in particular with rapid pressure build-up (TTL), and the drivemotor requires only a low power for rapid pressure build-up in a veryshort time (TTL=150 ms).

Also, because of the low flow losses of the advantageously configuredinlet valves, a pressure reduction can take place quickly via the inletvalves. Precise pressure reduction via the inlet valves EV can takeplace with corresponding control of the piston movement of the pressuresupply unit 20. Optionally, it is also possible to implement the knownMUX process with the valve circuit described above, or with pressurereduction control via outlet valves AV in one brake circuit, inparticular for consumers with low volume balance, e.g. the wheel brakeson the rear axle. In other words, a combination is also possible whichuses the MUX process in connection with the new valve circuit only intwo wheel brakes (e.g. front axle), and the pressure reduction takesplace conventionally on two further wheel brakes. This would mean thattwo wheel brakes/actuators are provided with inlet and outlet valves(EV+AV) and two wheel brakes/actuators only with inlet or switchingvalves EV. In this case, only the wheel brakes of the front axle areequipped with the new valve circuit according to the invention as shownin FIGS. 1a and 1 b, and a standard circuit/standard valves are used onthe rear axle.

FIG. 2a shows the pressure-volume curve of the wheel/wheel cylinder withconnecting lines as far as the switching valve SV and pressure sensor.Two curves are shown. Curve P_(aufa) corresponds to a so-called stiffcurve, the other curve P_(auf) requires substantially more volume. Thismay in extreme cases cause vapour bubbles due to e.g. play or poorpurging.

This means that the values for V_(a) e.g. for ΔP=P₁−P₂ are equal toV₁−V₂=ΔV_(a)=ΔS_(a) and at V_(auf)=A_(p) equal to V_(1a)−V_(2a)=ΔV=ΔS.This curve for p_(auf) and p_(ab) is stored e.g. for the first time online-end tests in the memory of the control device both for theindividual wheel brakes and for the brake circuits for both p_(auf) andp_(ab). On each braking, the curve is measured by comparison of pressureP with the volume V_((ΔS)). If a great deviation occurs, with astationary vehicle, the curves can be recorded or adapted as in theabove-mentioned test. It is also significant that the values canfluctuate between P_(auf) and P_(ab). It is normal that due to play, V₀is greater on pressure build-up P_(auf) but not on pressure reductionP_(ab). When the play has been eliminated, the curves are almost equal.

With poor purging or vapour bubbles, the curves behave similarly butwith greater volume for the corresponding pressure value.

For regulation, the p-V curves are used for pressure build-up P_(auf)and pressure reduction p_(ab).

FIG. 2b describes the simplified relevant pressure-volume curves forpressure regulation without hysteresis in a closed brake circuit orshifting after pressure reduction with opened outlet valve AV. Startingfrom a pressure p1, by defining a nominal differential pressure Δp, thenecessary volume shift ΔV or travel change Δs of the piston can be readfrom the curve. These differ and are dependent on whether the pressureis changed in one or more brake circuits. The piston is then movedaccordingly. If the pressure is reduced via one or more outlet valves,there is a volume loss in the pressure-generating unit. For furtherpressure reduction or pressure build-up in the closed brake circuit, thetravel allocation of the pressure-volume curve is determined bydetecting the pressure. This is required in regulation for monitoringthe volume balance, since the working chamber of the pressure-generatingunit only has a limited volume and thus towards the end of the pistonstroke, the piston would travel to the stop. If the piston of thepressure-generating unit travels close to the stop after a pressurechange and a further pressure rise is impending, the piston is retractedbriefly with closed switching valves SV in order to draw in volume fromthe storage container. In the design with double-stroke piston (FIG.5-6), this is retracted or switched to return stroke mode.

FIG. 3 shows a time development of the MUX regulation as known from WO2006/111393 A1 or WO 2010/091883 A1. This system is known as the4-channel MUX in which, except in the critical case of simultaneouspressure reduction (simultaneous p_(ab)), the pressure reduction p_(ab)is processed serially per wheel channel (cylinder). In the worst casescenario, this leads to a great delay time which causes large speeddifferences or even slip because of the individual response times of thevalve, motor and the time for the respective pressure reduction p_(ab).This both reduces the stability of the braking and disadvantageouslyextends the braking distance. Optimisation is performed on the responsetime of the switching valves, motor and pressure reduction gradients.Costs however limit optimisation. The case of simultaneous pressurereduction p_(ab) for all channels however occurs rarely in practice.

A further restriction exists in the regulation concept known from WO2006/111393 A1 or WO 2010/091883 A1 in the necessary priority for thepressure reduction p_(ab). If a pressure reduction is required, nopressure build-up p_(auf) can take place. Since usually the time forpressure build-up p_(auf) in the regulation cycle is around 200 ms, andtwo or three small p_(auf) take place per control cycle each withapproximately 10 ms delay time, this was not considered critical but isnoted as a minor defect of the 4-channel MUX.

The brake system according to the invention with its regulation conceptoffers the following improvements:

-   -   introduction of an additional outlet valve on the front axle;    -   various control methods and strategies for control and        regulation of the wheel brakes, e.g. VA, corner braking;    -   possibility of pressure build-up p_(auf) with simultaneous        pressure reduction p_(ab) (described in FIG. 5b and FIG. 5c ).

FIG. 3 shows the pressure curve in the individual wheel brakes over timefor conventional MUX mode in which the pressure in the wheel brakes isreduced temporally successively. V1 and V2 are the front wheel brakes,H1 and H2 the rear wheel brakes. At X, the signal is given forsimultaneous pressure reduction p_(ab). The response time t_(v)SV of theswitching valve SV is around 5 ms. The response time t_(v)M of the motoris around 10 ms. It is taken into account here that the pistons of thepressure-generating unit must first be positioned at (1) for thedifferent pressure levels of the individual wheels before the pressurechange. Then the pressure reduction p_(ab) takes place, assuming t_(ab)of around 10 ms, wherein during this time the pressure is reduced by Δpequal to around 20 bar.

The response times for the switching valves and motor assumed in FIG. 3are also assumed, for an objective comparison, for the depiction of thetemporal pressure development in FIG. 3a which corresponds to thepressure development according to the invention.

With conventional ABS systems with 4 inlet and 4 outlet valves, the ABSregulator always determines a Δp and then determines the time for whichthe outlet valve must be opened in order for the required pressurereduction to take place in the wheel brake. This time control is knownto be subject to tolerances, which limits the precision of the pressureregulation. Also, on closure of the outlet valve AV, pressurefluctuations always occur which cause disadvantageous noise.

The ABS regulator here determines the necessary pressure difference Δpsubstantially from the wheel angular acceleration and partially from thewheel slip, with correction factors for a) wheel inertia moment, b) gearstage and c) fading detection.

In contrast to time control, in conventional MUX as shown in FIGS. 1 and1 a, a volume control of the pressure supply is used in which Δv=Δp,wherein this takes place with evaluation of the pressure-volume curve ofthe wheel. Thus the precision of the pressure regulation is muchgreater, and the temporal pressure development can be influenced towardsthe end of the pressure reduction so that only slight pressurefluctuations occur.

At V1 in FIG. 3, after (X), the response time of t_(v)M and t_(v) of SV1acts. After opening of the switching valve SV1, the motor M is able toreduce the pressure over the time t_(ab). Then SV1 is closed again at(2). First however, the motor has already reached the required pressurelevel via the described volume control of the pressure supply.

Then the motor already begins the pressure reduction p_(ab) of the frontwheel V2, which takes place after opening of SV2 of V2. Thus thesequence V1-H2 has a total delay time of 60 ms with the aboveassumptions. This corresponds approximately to a regulation deviation Δvof around 15 km/h.

FIG. 3a shows the temporal pressure curve for the regulation conceptaccording to the invention. The pressure reduction p_(ab) at the frontwheel brake V1 at time (1) corresponds to (1) in FIG. 3. At the frontwheel brake V2, the additional outlet valve AV is used for pressurereduction p_(ab). The pressure reduction p_(ab) takes place almostwithout delay at (11) via a time control Δt described above, at which at(12) after closure of SV2 the pressure fluctuation occurs. Thusregulation of the front wheels V1 and V2—which at high μ make aconsiderably greater contribution to the brake force effect than therear wheels—is almost not delayed. During pressure reduction p_(ab) ofV1, the motor at (13) is already prepared for pressure change at therear wheel H1. This take place at (14) to (15) by volume control bymeans of the pressure-generating unit DE. Since the rear wheels H1, H2often have the same higher pressure level than V1 or V2, a simultaneouspressure reduction p_(ab) can take place by volume control.Alternatively, at H2 at (16) a time-controlled pressure reduction p_(ab)into the brake circuit can take place. One condition for this is thatthe MUX pressure level in the wheel brake of the rear wheel H2 is lowerthan at the rear wheel H1, at which the pressure reduction takes placeby means of volume control. It is also possible that the pressurereduction in the rear wheel brake H2 takes place by time-controlledopening of the associated outlet valve.

Variants of the front axle and rear axle regulation are shown in detailand described in FIGS. 4, 4 a, 4 b. A comparison of the delay times tvin relation to the conventional MUX methods in FIG. 3 show a significantimprovement at (10) with 60 ms and (17) with 25 ms. This is possible bythe use of the time-controlled pressure reduction p_(ab) at V2 and H2 bymeans of outlet valve or into the brake circuit, partially simultaneousp_(ab) at H1 and H2, and priority control at H1 and (13).

FIGS. 4-4 b show the pressure development with different actual pressurelevels and variants of the front and rear wheels H1, H2, V1 and V2 withthe regulation concept according to the invention.

FIG. 4 shows the temporal pressure development V1 to H2 in differentphases. Phase 0-X shows a pressure development in which, because of thedifferent pressure levels in the wheel brakes, no simultaneous pressurebuild-up p_(auf) and pressure reduction p_(ab) can take place, which ismost often the case. Consequently, here also full multiplex mode isactive i.e. precise Δp regulation via volume control both for pressurebuild-up p_(auf) and for pressure reduction p_(ab). At pressure build-upp_(auf), sometimes e.g. at 20 with simultaneous pressure build-uprequirement, a temporally offset pressure build-up p_(auf) takes placeat H1 and H2. However also a partially simultaneous pressure build-upP_(auf) may take place. Also, a partially simultaneous pressurereduction is possible as shown in FIG. 4 b.

At X in FIG. 4, the signal p_(ab) for simultaneous pressure reduction inall wheel brakes is given, which is implemented without time delay.These two variants A and B are shown in more detail in FIG. 4a and FIG.4 b.

FIG. 4a shows the variant A, again starting at point X, with p_(ab) atV1 and V2 as described in FIG. 3a . At the front wheel V1, the pressurereduction p_(ab) takes place by means of volume control by thepressure-generating unit DE. For the rear wheel H2, at 21 a controlledp_(ab) takes place over time dt=f(dp). This pressure reduction takesplace when a sufficient differential pressure Δp between H2-V1 ispresent. The pressure levels of all wheels are known in the regulationconcept according to the invention or MUX method, so that a relativelyprecise pressure reduction p_(ab) in the rear wheel H2 is achieved bythe time control or opening of the switching valve SVH2. The necessaryopening time Δt may, because of the change in pressure level, be adaptedflexibly by M1 (MUX). The pressure reduction p_(ab) of the rear wheel H1also takes place via volume control, starting from the preparation at13, and then by opening of the associated switching valve at 22.

As a result, there is a relatively small tvmax as described in FIG. 3a .At 11, the time-controlled pressure reduction p_(ab) for the front wheelV2 takes place. Here again, the pressure difference from the storagecontainer is known and hence a precise pressure control is possible bythe time-controlled opening of the outlet valve.

FIG. 4b shows variant B for partially simultaneous pressure reduction atthe rear wheels H1 and H2, starting from a relatively small pressuredifference between the rear wheels H1 and H2. Here, after preparation at13, at 23 the pressure reduction p_(ab) takes place with MUX, i.e.volume control by means of the pressure-generating unit. At 24, the rearwheel H2 is switched to pressure reduction p_(ab) by opening of theswitching valve SVH2 assigned to the rear wheel H2. At 25, thecontrolled pressure reduction p_(ab) for H2 is achieved, so theswitching valve SVH2 is closed. At 26, via the volume control, the Δpfor the rear wheel H1 is reached, so at 26 the switching valve SVH1 isclosed.

Both methods allow a short delay time. In some cases, the controlledpressure reduction p_(ab) causes the pressure fluctuations, whichhowever only occur in extreme cases with simultaneous pressure reductionp_(ab).

To summarise and in addition, the following features apply:

-   -   the pressure of each wheel at the start and end of the pressure        reduction p_(ab) (FIG. 4, 4 a) is stored in the memory; the two        values are used as reference for the subsequent pressure changes        of the wheel or following wheels;    -   the pressure of the last pressure build-up p_(auf) (FIG. 4) is        stored in the memory and thus forms the basis for setting the        pressure of the pressure-generating unit DE in preparation for        the following pressure reduction p_(ab);    -   the outlet valves AV are time-controlled, wherein for this the        pressure difference from the storage container is taken into        account; one wheel of the front axle is connected to the brake        circuit only via switching valve SV, wherein a switching valve        SV towards the brake circuit and an outlet valve AV towards the        storage container are assigned to the second wheel of the front        axle, so that switching valves SV of the rear axle are        time-controlled to p_(ab), while MUX controls the wheels of the        front axle with low pressure level, in which the time control        t_(ab) of the switching valves SV of the rear axle HA evaluates        the differential pressure Δp;    -   as well as the differential pressure, for the time control        (valves are opened for a predefined time), the Δp of the        corresponding outlet valve AV is evaluated from the        pressure-volume curve. Priority control of the MUX with        orientation towards driving stability with braking distance,        e.g. wheel of the front axle VA has priority and also, at        positive μ jump with highest negative p_(ab) or positive        acceleration p_(auf), since here the Δp to be regulated is        greatest;    -   the pressure change by volume control by means of the        pressure-generating unit, and the time-controlled opening of        outlet valve or switching valve used in parallel, forms the        combined MUX regulator;    -   in time control, the corresponding volume must be taken into        account in the volume delivery, corresponding to the pressure        change Δp determined by the regulator.

FIG. 5 describes a further embodiment of the pressure supply unit DEaccording to the invention with pressure regulation with master brakecylinder, master brake cylinder valves HZV, pressure supply withdouble-stroke piston, switching and outlet valves.

The master brake cylinder HZE is connected to brake circuits BKI andBKII. For the separation logic, the same applies as in FIG. 1 a. Theadvance stroke chamber 4 of the double-stroke piston 3 is connected tobrake circuit BI via the isolating valve TV1, and to BK II via theisolating valve TV2. The return stroke chamber 4 a is connected via theisolating valve TV2 b to BK II and via HZE to BK I. The transmissionpreferably takes place by the floating piston SK. The advance strokechamber 4 and return stroke chamber 4 a can be connected togetherhydraulically via a switching valve ShV. This switching valve SvH allowsa short-circuit between the two chambers, and is used in particular onadvance stroke (towards the left) to reduce the hydraulically activearea of the piston 3. The return stroke chamber 4 a of DHK 3 isconnected via the switching valve PD1 to the storage container 10. Thetwo chambers 4 and 4 a are also each connected via a check valve to thestorage container 10. This system configuration offers the followingdegrees of freedom:

-   -   pressure build-up and pressure reduction in all brake circuits        with multiplex regulation (pressure regulation with        pressure-volume control) in all wheel brake cylinders        simultaneously or sequentially via isolating valves TV1, TV2 and        PD1, and the switching valves SV1-SV4 of the wheel brakes        RB1-RB4;    -   multiplex regulation in pressure build-up and pressure reduction        in brake circuit I and pressure reduction in brake circuit II        via outlet valves AV3, ZAV;    -   multiplex regulation with simultaneous pressure reduction in        brake circuit BK I and pressure build-up in BK II via        double-stroke piston control;    -   multiplex regulation with simultaneous pressure reduction in        brake circuit BK II and pressure build-up in BK I via        double-stroke piston control;    -   pressure reduction RB3 at any time via AV3 in multiplex        regulation.

FIG. 5a shows as an example some of the pressure regulationpossibilities which may take place temporally in parallel with eachother:

-   -   controlled pressure reduction in RB1 via SV1, TV1 by        pressure-volume control by means of return stroke of the        double-stroke piston 3 with open PD3 valve, or alternatively        pressure control of pressure reduction via pressure estimation        based on phase current measurement in brake circuit I;    -   controlled pressure reduction in RB2 via SV2, TV1 via        pressure-volume control by means of return stroke of        double-stroke piston 3 with open PD3 valve, or alternatively        pressure control of pressure reduction via pressure estimation        based on phase current measurement in brake circuit I;    -   pressure reduction in RB3 via AV3 with time control of outlet        valve AV3;    -   pressure reduction in RB4 by means of double-stroke piston 3        with time control of one of the switching valves SV4 or PD1,        wherein the other valve must also be opened at this time, or        alternatively pressure control of pressure reduction via        pressure estimation based on pressure measurement in brake        circuit II.

For simultaneous pressure reduction p_(ab) at different startingpressures, optionally a deviation may occur from the MUX regulation inthat the switching valves SV1 and SV2 are opened with time offset. Theisolating valve TV1 is here opened continuously on pressure reduction.Since a higher pressure prevails in RB1, the switching valve SV1 isopened before switching valve SV2. On the basis of knowledge of thepressure difference (wheel pressure RB1 and RB2 and pressure in theadvance stroke chamber of the pressure supply unit), the time controlcan be dimensioned precisely. If the pressure in the advance strokechamber of the pressure supply unit DE is not determined precisely,because at the same time a pressure reduction via ZAV is taking place inwheel brake RB4, and TV2 is closed, the pressure in the advance strokechamber may be used via pressure estimation p/i from the torque of theelectric motor. The switching valve SV2 is open when the pressure of thepressure-generating unit DE is approximately reached. The furtherpressure reduction then takes place simultaneously in both wheel brakecylinders by control via piston 3 with open SV1, SV2 and TV1. When thetarget pressure of a respective wheel is reached, the correspondingvalve SV1 or SV2 is closed. If further pressure reduction is required ina wheel, further pressure reduction can take place only in one wheelbrake.

In parallel to the pressure reduction control in MUX mode, in BK II thepressure can be reduced by time control of AV3. This can be determinedtemporally freely because closure of SV3 does not influence the otherwheel brake cylinders. Also, the temporal activation of the pressurereduction in wheel brake RB4 can be selected freely on pressurereduction of BK 1 in MUX mode.

FIG. 5b shows as an example some of the pressure regulationpossibilities which may take place temporally in parallel with eachother:

-   -   controlled pressure reduction in RB1 via switching valves SV1        and TV1 via pressure-volume control by means of return stroke of        double-stroke piston 3 with closed PD1 valve;    -   controlled pressure reduction in RB2 via switching valves SV2        and TV1 via pressure-volume control by means of return stroke of        double-stroke piston 3 with closed PD1 valve;    -   pressure reduction in RB3 via outlet valve AV3 with time control        Δt (opening of AV3 valve for period Δt);    -   pressure build-up in RB4 via isolating valve TV2 b (ZAV) with        pressure-volume control by means of return stroke of        double-stroke piston with closed PD1 valve.

For simultaneous pressure reduction and pressure build-up in wheel brakeRB4, the pressure build-up dynamic is determined by the pressurereduction dynamic and the effective piston area and hydraulicdifferential pressures. This must be taken into account in theregulation. When the target pressure is reached in wheel brake RB4, theswitching valve SV4 is closed. If the pressure in BKI is to be reducedfurther, PD1 is opened for further pressure reduction in brake circuitI.

FIG. 5c shows as an example some of the pressure regulationpossibilities which may take place temporally in parallel with eachother:

-   -   controlled pressure reduction in RB1 via switching valves SV1        and TV1 via pressure-volume control by means of advance stroke        of double-stroke piston 3 with open PD1 valve;    -   controlled pressure reduction in RB2 via switching valves SV2        and TV1 via pressure-volume control by means of advance stroke        of double-stroke piston 3 with open PD1 valve;    -   pressure reduction in RB3 via outlet valve AV3 with time control        of outlet valve AV3;    -   pressure reduction in RB4 via double-stroke piston 3 with time        control of switching valve SV4 or PD1 valve.

For the many functions of pressure reduction p_(ab) in one brake circuitand pressure build-up p_(auf) in the other brake circuit, it is possiblefor the floating piston SK of the master brake cylinder HZE to move. Toprevent this, a blocking element SE may be arranged in BK1 or BK2 whichacts directly on the SK as mechanical blocking. The blocking element mayalso be part of the HZV.

With this pressure regulation system, the functions described in 5 b and5 c of p_(auf) in one brake circuit and p_(ab) in the other can beimplemented independently of the pressure level of the brake circuits.

FIG. 5d shows as an example the pressure reduction in brake circuit Iand brake circuit II which is implemented on pressure reduction fromhigh pressures. With isolating valves TV1 and TV2 open, the following iscarried out:

-   -   controlled pressure build-up in RB1 by time control of valves        SV1 and PD3 via pressure control of the pressure reduction by        pressure estimation based on phase current measurement in brake        circuit I;    -   controlled pressure build-up in RB2 by time control of valves        SV2 and PD3 via pressure control of the pressure reduction by        pressure estimation based on phase current measurement in brake        circuit I;    -   controlled pressure build-up in RB3 by time control of valves        SV3 and PD3 via pressure control of the pressure reduction by        pressure measurement based on the pressure sensor in brake        circuit II;    -   controlled pressure build-up in RB4 by time control of valves        SV4 and PD3 via pressure control of the pressure reduction by        pressure measurement based on the pressure sensor in brake        circuit II.

For pressure reduction for individual wheels, in the same way as shownin FIG. 5a , the switching valves SV1-SV4 may be switched with atemporal offset.

One possibility (not shown) is that of pressure reduction via PD1 valvewhich is similar to the process for PD3 valve. The pressure reductionmay take place for all brake circuits via the PD1 valve. The pressurereduction may also take place via PD3 and PD1 valve. This is similar toFIG. 5a with the difference that the pressure of all wheel brakes isreduced via the pressure supply unit, and hence the advantages ofpressure reduction in the closed brake circuit are obtained, which hassafety advantages in particular after completion of a braking process(e.g. after ABS operation).

FIG. 6 describes a system with double-stroke piston in advantageous dualcircuit design. The structure of THZ, DE and the valve circuits forpressure control ABS with MUX and AV is identical to FIG. 5 c.

in contrast to FIG. 5c , the pressure supply in the advance stroke actson brake circuit II and the back of the floating piston SK. Thistransmits the volume and pressure to brake circuit I. If thedouble-stroke piston has travelled close to the end position, it isreversed and operated in return stroke and acts on BK I. Then via thereturn stroke, pressure acts on the front side of the floating pistonSK. This transmits the pressure again to brake circuit BK II. The SKpiston is always active with its seals, as in the current THZ.

The double-stroke piston 3 also has a bypass valve ShV, which isswitched substantially under three conditions:

-   -   a) at high pressure, to reduce the piston force, the volume of        the advance stroke is also conducted to the back of the        double-stroke piston 3 to balance the pressure;    -   b) in ABS regulation and also MUX regulation, the double-stroke        piston 3 is switched in a single circuit via the ShV valve;    -   c) pressure reduction p_(ab) from high pressure level takes        place simultaneously in both brake circuits BK I and BK II.

This valve circuit has the consequence for the floating piston positionthat the return spring 1 moves the floating piston SK to the right stopor locks it in the middle position. The pressure sensor 9 measures thepressure in BK II and, with a “single circuit” arrangement for theregulation and control functions, can evaluate the pressure in bothbrake circuits.

For special functions with pressure build-up P_(auf) in BK I andpressure reduction p_(ab) in BK II and vice versa, it is advantageous toplace in the connection to THZ in BK II, or a blocking element SE inbrake circuit BK 1 which prevents the movement of the floating pistonSK. The blocking valve SE may also be part of the HVZ.

This system contains the additional potential for reducing the pressurein BK II via the DHK piston 3, and separately from BK I via the valvesTV2 b and PD1. This solution has advantages in use for differentpressure level activation on the two axles in recuperation. For this,the blocking element SE must then be used at SK or in BK I.

The functions described in FIGS. 5-6 and the additional overlaid timecontrol via outlet valve(s) give the MUX system according to theinvention a very good performance with high regulation dynamic andprecision, with significantly lower cost than individual wheelregulation with inlet and outlet valves.

1. A brake system for motor vehicles, the brake system including: anactuation device, a travel simulator configured to generate a feedbackforce on the actuation device, a first piston-cylinder unit, having atleast one piston that separates two working chambers from each other,wherein each working chamber is connected via at least one hydraulicconnecting line to at least one wheel brake of a brake circuit, whereinat least one wheel brake is assigned to each brake circuit and eachwheel brake is enabled to be connected to an associated one of thehydraulic connecting lines via an associated controllable switchingvalve, a control device, at least one pressure supply unit configured tobe driven by an electric motor and having at least one working chamber,wherein the at least one pressure supply unit is configured to build upand/or reduce brake pressure in one or more of the wheel brakessimultaneously or successively, and at least one outlet valve, wherein arespective one of the at least one outlet valve is associated only witha single wheel brake or is associated with one wheel brake of arespective brake circuit, wherein the outlet valve is arranged in ahydraulic connection between the wheel brake with which it is associatedand a pressure medium storage container and wherein no further valve isarranged between the respective one of the at least one outlet valve andthe pressure medium storage container.
 2. The brake system according toclaim 1, wherein a respective one of the at least one outlet valve isassociated with a front wheel brake.
 3. The brake system according toclaim 1, wherein the pressure supply unit has only one working chamber,wherein the one working chamber is connected by means of two furtherhydraulic connecting lines to the respective hydraulic connecting linesof the respective brake circuits or to a working chamber of the firstpiston-cylinder unit, the brake system further including at least oneisolating valve arranged in at least a respective one of the two furtherhydraulic connecting lines.
 4. The brake system according to claim 1,wherein the control device has a memory which contains an actual wheelbrake pressure set in a respective wheel brake, wherein, by means of thestored actual wheel brake pressure and a nominal wheel brake pressure,the control unit is configured to determine by taking into account atleast one pressure-volume curve of the one or more of the wheel brakes,a calculated time for which a respective outlet valve is to be opened inorder to reach the nominal wheel brake pressure in the one or more ofthe wheel brakes, and wherein the control device is configured to closethe respective outlet valve after expiry of the calculated time.
 5. Thebrake system according to claim 4, wherein the control device isconfigured to reduce brake pressure in the one or more wheel brakes withwhich an outlet valve is/are associated, via the associated outletvalve, which is opened for the precalculated time, into the pressuremedium storage container, wherein, for simultaneous pressure reductionor pressure build-up another wheel brake of a given one of the brakecircuits and/or at least one further wheel brake of another one of thebrake circuits, the control device opens the respective associatedswitching valve(s) and sets or regulates the nominal brake pressure inthe respective associated wheel brake by means of correspondingactivation of the pressure supply unit.
 6. The brake system according toclaim 1, wherein an outlet valve associated with a wheel brake of one ofthe brake circuits serves for pressure reduction in another one of thewheel brakes of the one of the brake circuits, wherein for commonpressure reduction in one of the brake circuits, the control device isconfigured to open both the switching valves and the respective outletvalve associated with the one of the brake circuits.
 7. The brake systemaccording to claim 6, wherein during opening of the switching valvesassociated with the one of the brake circuits, the control device isconfigured to separate the one of the brake circuits from the pressuresupply unit by closing an associated isolating valve or by locking thepiston of the first piston-cylinder unit.
 8. The brake system accordingto claim 1, wherein, for simultaneous or temporally offset pressurereduction and/or pressure build-up using of the pressure supply unit,the control unit is configured to set a pressure in its at least onepressure chamber (4, 4 a) and to open and/or close the switching valvessimultaneously and/or with temporal offset in order to set the nominalpressure required in the respective wheel brakes, wherein, by means ofthe at least one outlet valve, pressure is reduced in the wheel brake orbrakes associated with the at least one outlet valve, independently ofthe pressure supply unit.
 9. The brake system according to claim 1,wherein for simultaneous and/or temporally offset pressure reductionand/or pressure build-up, the control device is configured to actuatethe pressure supply unit, taking into account wheel brake pressurescalculated or prevailing in the wheel brakes, wherein the pressuresupply unit is configured to generate a nominal pressure to be set for arespective wheel brake with a respective open switching valve, and afterreaching the nominal pressure, the control device is configured to closethe respective switching valve to maintain the nominal pressure in therespective wheel brake, wherein the at least one outlet valve isconfigured to enable pressure to be reduced in a wheel brake associatedwith a respective one of the at least one outlet valve independently ofthe pressure-generating unit.
 10. The brake system according to claim 1,wherein at least one pressure sensor serves to determine the pressure inat least one brake circuit.
 11. The brake system according to claim 1,wherein the control device has a memory configured to store actual wheelbrake pressures set in each wheel brake and/or probable wheel brakepressures of each wheel brake continuously calculated and updated withmeasured values in a control model, wherein, for simultaneous pressurereduction in at least two wheel brakes of one brake circuit, todifferent nominal pressures in the at least two wheel brakes, thecontrol unit is configured to evaluate a pressure-volume curve of eachof the at least two wheel brakes of the one brake circuit and, using alowest nominal pressure to be generated, is configured to calculate apiston travel of a piston of the pressure supply unit required and tomove the piston of the pressure supply unit by the piston travel bymeans of the electric motor of the pressure supply unit, wherein aswitching valve of a particular wheel brake with the lowest nominalpressure remains open until the nominal pressure has been set in theparticular wheel brake; and wherein, for the switching valves of thewheel brakes not having the lowest nominal pressure, the control deviceis configured to individually calculate temporal durations for which theswitching valves of the wheel brakes not having the lowest nominalpressure are to remain open so that the pressures in the respectivewheel brakes are reduced to respective nominal pressures, and to openthe switching valves only for the respective calculated temporaldurations.
 12. The brake system according to claim 11, wherein theswitching valve of the particular wheel brake is opened first at a timeat which pressure in an associated brake circuit is equal to or higherthan an actual pressure in the particular wheel brake.
 13. The brakesystem according to claim 10, wherein at the same time as, or temporallyoverlapping, the pressure reduction in the wheel brakes over the brakecircuit(s), the pressure is reduced in at least one further wheel brakeby opening of an associated outlet valve.
 14. The brake system accordingto claim 1, wherein the control device has a memory configured to storean actual brake pressure set in each wheel brake and/or probable wheelbrake pressures of each wheel brake continuously calculated and updatedwith measured values in a control model, wherein for simultaneouspressure build-up in at least two wheel brakes of one brake circuit, todifferent nominal pressures in the wheel brakes, the control unit isconfigured to evaluate a respective pressure-volume curve of each of thewheel brakes concerned and to use a highest nominal pressure to begenerated to calculate a piston travel of a piston of the pressuresupply unit required for this and to move the piston of the pressuresupply unit by the piston travel, using the electric motor of thepressure supply unit, wherein a switching valve of a particular wheelbrake with a highest nominal pressure remains open until the nominalpressure has been set in the particular wheel brake; and wherein, forthe switching valves of the wheel brakes not having the highest nominalpressure, the control device is configured to individually calculatetemporal durations for which the switching valves of the wheel brakesnot having the highest nominal pressure are to remain open so that thepressure in the respective wheel brakes not having the highest nominalpressure is built up to respective nominal pressures, and to open therespective switching valves only for the respective calculated temporaldurations.
 15. The brake system according to claim 14, wherein theswitching valve of the particular wheel brake is opened first at a timeat which pressure in an associated brake circuit is equal to or lowerthan an actual pressure in the particular wheel brake.
 16. The brakesystem according to claim 14, wherein at the same time as, or temporallyoverlapping, the pressure build-up in the wheel brakes over the brakecircuit(s), the pressure is reduced in at least one further wheel brakeby opening of an associated outlet valve.
 17. The brake system accordingto claim 1, wherein the pressure supply unit is a piston-cylinder systemhave a single piston, wherein the single piston is driven by theelectric motor and delimits at least one working chamber.
 18. The brakesystem according to claim 12, wherein a piston of the pressure supplyunit separates a first working chamber and a second working chambertightly from each other, wherein both the first and the second workingchamber are enabled, by corresponding actuation of isolating valves, tobe used for simultaneous and/or temporally offset pressure build-up andpressure reduction in the wheel brakes of the brake circuits.
 19. Thebrake system according to claim 1, wherein, for pressure build-up and/orpressure reduction in at least one wheel brake, the control device isconfigured to evaluate a pressure-volume curve of each wheel brake andfrom a respective pressure rise or fall to be generated, to calculatepiston travel of a piston of the pressure supply unit required for therespective pressure rise or fall to be generated, and to correspondinglyactuate required valves, and to move the piston of the pressure supplyunit by an amount of the piston travel, using the electric motor. 20.The brake system according to claim 1, wherein the control device isconfigured to determine a pressure in one brake circuit by means of apressure sensor, and to determine a pressure in another brake circuitvia a phase current of the electric motor of the power supply unit, andto take into account determined or calculated brake circuit pressure incontrolling pressure build-up or pressure reduction in the wheel brakes.21. The brake system according to claim 1, wherein the switching valvesare digital valves that are open in an unpowered state.
 22. The brakesystem according to claim 1, wherein an interior or armature housing ofan inlet valve of a respective wheel brake is connected via a hydraulicline to an associated actuator/brake circuit that includes therespective wheel brake, and wherein a valve seat outlet of the inletvalve is connected via a hydraulic line to the respective wheel brake.23. The brake system according to claim 1, wherein a working chamber ofthe pressure supply unit is connected via a hydraulic connection to thepressure medium storage container, the brake system further including aswitchable valve configured to shut off the hydraulic connection suchthat a pressure reduction in at least one wheel brake is enabled to takeplace time-controlled by opening the associated switching valve of theat least one wheel brake and by opening the switchable valve, wherein apredefined opening time determines the wheel brake pressure to be set.24. The brake system according to claim 23, wherein, via anotherpressure chamber of the pressure supply unit, by means of the pressuresupply unit, a volume-controlled pressure reduction or pressure build-upis enabled to take place in at least one wheel brake when the switchingvalves of the respective wheel brakes are open simultaneously.
 25. Thebrake system according to claim 1, wherein the pressure supply unitcomprises one pressure chamber, wherein the one pressure chamber isconnected via a hydraulic connection to a working chamber of the firstpiston-cylinder unit, wherein the hydraulic connection is enabled to beshut off by movement of the piston of the first piston-cylinder unitinto a shut-off position.
 26. The brake system according to claim 1,wherein, on pressure build-up and/or on pressure reduction, the controldevice is configured to take into account an absolute position of apiston of the pressure supply unit and to determine a necessary movementtravel of the piston of the pressure supply unit, depending on actualpressures, nominal pressures and pressure-volume curves of therespective wheel brakes involved in the pressure build-up or reduction.27. The brake system according to claim 26, wherein, in normal operationwith normal brake force amplification, recuperation, and anti-lockbraking system (ABS), at low μ, pressure is built up and reducedsimultaneously or sequentially in all wheel brakes via the movementtravel of the piston of the pressure supply unit, taking into accountthe respective pressure-volume curves, and wherein, in operatingsituations with ABS at high μ, μ split, electronic stability program(ESP), and anti-slip regulation (ASR), pressure in at least one wheelbrake is reduced via at least one associated outlet valve and/or via oneor more outlet valves of the pressure supply unit at the same time. 28.The brake system according to claim 1, wherein, by means of theactuation device, upon failure of the pressure supply unit or uponoccurrence of another fault, the at least one piston of the firstpiston-cylinder device is enabled to be set to build up pressure in atleast one wheel brake.
 29. The brake system according to claim 1,wherein pressure reduction takes place via a travel-controlled stroke ofa double-stroke piston of the pressure supply unit or via a connectingline of a pressure chamber of the double-stroke piston with an openvalve into the storage container, wherein the control device isconfigured to use a pressure measured in a respective brake circuit or apressure calculated to control the pressure reduction.
 30. The brakesystem according to claim 1, wherein a pressure reduction takes place athigh pressures of approximately 200 bar down to pressures in a rangeclose to blocking pressure in normal operation, of approximately 80-100bar, via pressure or time control of a switching valve disposed in ahydraulic line between a working chamber of the pressure supply unit andthe storage container, with optionally simultaneously or temporallyoffset travel control of a double-stroke piston of the pressure supplyunit in advance stroke mode and subsequent pressure reduction toatmospheric pressure by pressure-volume control in return stroke mode ofthe double-stroke piston.
 31. The brake system according to claim 18,wherein the isolating valves are designed for high throughflow and lowdifferential pressures.
 32. The brake system according to claim 31,wherein, by means of the pressure supply device before or during openingof an isolating valve, the control unit is configured to set a pressurein a corresponding pressure chamber of the pressure supply unit by amovement of the piston of the pressure supply unit that is so great thata sufficiently small differential pressure is created at the isolatingvalve to be opened.
 33. A method of testing tightness and movability ofa floating piston of a master brake cylinder of a brake system, whereinthe master brake cylinder has a radially running channel in its cylinderwall which opens into a first pressure chamber of the master brakecylinder, wherein a mouth opening of the channel is configured to be isclosed by the floating piston as soon as the floating piston has beenmoved out of its normal position by an amount corresponding to adiameter of the mouth opening, wherein the channel is hydraulicallyconnected to a pressure chamber of a pressure supply unit, wherein amechanical stop is arranged in the master brake cylinder and a spring isconfigured to force-load the floating piston in a direction of the stop,wherein the floating piston is in its normal position when it lies onthe stop, the method including: a. building up a pressure, via thepressure supply unit, a in first pressure chamber and in a secondpressure chamber of the master brake cylinder, and switching valves suchthat the pressure in the second pressure chamber of the master brakecylinder is greater than in the first pressure chamber of the masterbrake cylinder, such that the floating piston moves and closes thechannel, thereby closing off the connection between the channel and thepressure supply unit, with open switching valves associated withrespective wheel brakes in a first brake circuit and with closedswitching valves associated with respective wheel brakes in a secondbrake circuit; b. producing, by means of the pressure supply unit, atest travel profile of the floating piston and/or a temporalpressure-volume curve in one of the pressure chambers of the masterbrake cylinder; c. using a signal from a pressure sensor, comparing apressure rise determined from said producing, to a nominal value curvewith open switching valves associated with respective wheel brakes, andevaluating a comparison result.
 34. The method of testing the tightnessand movability of the floating piston of the master brake cylinder of abrake system according to claim 33, wherein the method further includes:a) storing pressure in the second brake circuit by closure of the valvesof the second brake circuit during a parking stop with a vehicleincorporating a brake system comprising the components found in claim 33stationary; b) subsequently, reducing pressure in the first brakecircuit, by means of the pressure supply unit, to 0 to 1 bar; c)subsequently blocking the pressure supply unit, such that no furtherpiston movement of the piston of the pressure supply unit takes place,wherein a control device closes an isolating valve disposed in ahydraulic connecting line between the pressure supply unit an the secondbrake circuit; d) then, opening the switching valves of the second brakecircuit, whereby because of the pressure on a back side of the floatingpiston of the master brake cylinder, the floating piston moves and thusbuilds up pressure in the first brake circuit until a pressureequilibrium exists between the first and second brake circuits; and e)measuring pressure in at least one of the first or second brake circuitsby means of a pressure sensor in at least one of the first or secondbrake circuits, and evaluating a pressure curve.
 35. A method ofreducing pressure at greater than or equal to 100 bar in a brake systemwith: an actuation device, a travel simulator configured to generate afeedback force on the actuation device, a first piston-cylinder unit,having at least one piston that separates two working chambers from eachother, wherein each working chamber is connected via at least onehydraulic connecting line to at least one wheel brake of a brakecircuit, wherein at least one wheel brake is assigned to each brakecircuit and each wheel brake is enabled to be connected to an associatedone of the hydraulic connecting lines via an associated controllableswitching valve, a control device, at least one pressure supply unitconfigured to be driven by an electric motor and having at least oneworking chamber, wherein the at least one pressure supply unit isconfigured to build up and/or reduce brake pressure in one or more ofthe wheel brakes simultaneously or successively, and at least one outletvalve, wherein a respective one of the at least one outlet valve isassociated only with a single wheel brake or is associated with onewheel brake of a respective brake circuit, wherein the outlet valve isarranged in a hydraulic connection between the wheel brake with which itis associated and a pressure medium storage container, and wherein nofurther valve is arranged between the respective one of the at least oneoutlet valve and the pressure medium storage container, wherein themethod includes: a. reducing pressure via pressure control and pressuremeasurement via a valve disposed between a working chamber of thepressure supply unit and the pressure medium storage container andopening of the switching valves to the respective wheel brakes, b.moving a double-stroke piston of the pressure supply unit in advancestroke mode with simultaneous opening of one or more valve(s) thatconnect the two working chambers of the first piston-cylinder unit; c.further reducing pressure by means of the double-stroke piston in returnstroke mode via pressure-volume control with pressure measurement via apressure sensor; and d. positioning the double-stroke piston in astarting position corresponding to an initial position for atmosphericpressure, and subsequently delivering of hydraulic medium to thepressure supply unit via check valves.